Steerable antenna array



L. RUSSELL STEERABLE ANTENNA ARRAY Jan. 17, 1961 5 Sheets-Sheet 1 Filed Aug. 24. 1954 FIGHA INVEN'TOR. LlnV/Jay R B 4 ay/w W 7 dz/JIM Jan. 17, 1961 RUSSELL STEERABLE ANTENNA ARRAY 5 Sheets-Sheet 2 Filed Aug. 24. 1954 20Q&

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STEERABLE ANTENNA ARRAY Filed Aug. 24, 1954 5 Sheets-Sheet 3 INVENTOR. L/QC/Soj us:e

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7 STEERABLE ANTENNA ARRAY Filed Aug. 24, 1954 5 Sheets-Sheet 4 Jan. 17, 1961 1.. RUSSELL 2,968,308

STEERABLE 'ANTENNA ARRAY Filed Aug. 24. 1954 s Sheets-Sheet 5 o A T l I i 0 0 Q. Q.

Cr 5 o INVENTOR. Lmels o 3 Rune 2,968,808 Patented Jan. 17, 1961 ice STEERABLE ANTENNA ARRAY Lindsay Russell, Needham, Mass.,' assignor to Andrew Alford, Boston, Mass.

Filed Aug. 24, 1954,'Ser. No. 451,754

Claims. (Cl. 343-854) The present invention relates to an antenna system for use both in transmission and reception, in which an antenna array having a definite combination and placement of units is so controlled for transmission or reception as to provide a directional transmission and receiving system wherein for transmission a radio controlled beam may be turned and directed about the entire azimuth with very great accuracy and with the beam very directive. When used for reception, the array receives a radio wave and the direction of the source is determined in exactly the reverse fashion in which transmission is accomplished.

In the present system the beam may be equally well directed for any heading in a complete 360 azimuth. The invention further relates to an electrical phase shifting system and pertains more particularly to the use of a plurality of variable delay lines to control the relative phase at which radio frequency currents are fed to different elements of the antenna array.

In the present invention, the direction of the radiation of the antenna system is controlled continuously through the use of a continuously variable delay line, in which no transmission line switch is used such as is ordinarily required to change the amount of delay in the line where the delay for instance is changed in discreet steps.

The invention further provides means for mechanically controlling the instantaneous delay of a plurality of delay lines by one motor and a gear train common to the entire assembly.

A further advantage of the present invention is that the terminal behaviour of each delay line is very much like that of a length of coaxial cable in that it behaves like an unbalanced transmission line of approximately constant characteristic impedance regardless of the delay setting and frequency.

For instance the delay line may permit a time delay variation of 0.15 micro-second or more at all frequencies below 35 mega cycles, and yet may be physically contained in a unit whose longest dimension is three feet. The method of varying delay involves no switching and involves no sliding electrical contacts and is not in consequence attended by the generation of any electrical noise which may impair the operation of the radio receiving system.

In the present invention the antenna system preferably consists of a circle of individual radiating elements spaced equi-angular about a circumference and connected to a central control housing wherein a delay line is connected to each radiating element by a feed cable, the delay lines being operated and controlled by a single element designed to compensate for the maximum delay which is equivalent to the diameter of the circle on which the radiating elements are placed.

In the preferred form of the invention, two sets of radiating elements are used, one comprising units placed on an outer circumference and another comprising units placed on an inner circumference. In this case there is a delay line for each of the radiating elements of the outer circle and a delay line for each of the radiating elements of the inner circle and a single unit is used to control the both groups of units together.

In a system which has been built and is in operation, there are 16 units in the inner circle and also 16 units in the outer circle, preferably staggered or positioned with equal angular spacings. In this case each unit is angularly placed at Il /1 about the center of the circles which are preferably concentric one with the other. All of the delay lines are mounted on a single frame and each is controlled by a linkage system which is driven from two common driving ring means through which all the delay lines are controlled to govern their setting in any azimuth direction.

Other advantages and improvements in the system of the present invention will be more fully understood from the description in the specification set forth below when taken in connection with the drawings, illustrating the same, in which:

Figure 1 shows the basic circuit for an artificial delay line of the present invention.

Figure 2 shows a few sections in circuit shown in Figure 1.

Figure 3 shows a modification of the circuit of Figure 1 in which compensation is made for phase distortion.

Figure 4 shows a physical arrangement of the elements of Figure 3 in a maximumdelay position.

Figure 4a shows the circuit of Figure 4 with the delay line providing a minimum delay position.

Figure 5 shows a plane view of an array of antenna elements such as are used and whose radiation direction is controlled in the present invention.

Figure 6 shows an arrangement of delay lines in perspective controlled simultaneously by a common mechanism for operating the same to control the direction of reception or radiation of the antenna system.

Figure 7 shows a three bar linkage system used to rotate the delay line control shaft for setting the delay line to conform to the direction of reception or radiation of the antenna units.

Figure 8 shows a mechanism suitable for generating a circular motion at one end of each of a group of the three bar linkage arrangements used in Figure 7.

Figure 9 shows a transformer circuit used for energizing the delay lines for supplying the radio frequency power.

Figure 10 shows the adjustment of the linkage arms shown diagrammatically in Figure 7, and

Figure 11 shows a side view of the delay line with the outer cover removed.

Figure 12 shows a plan view of the controls for operating the delay line, and

Figure 13 shows a perspective view of the transformer and power circuit for energizing the lines.

In the basic circuit illustrated in Figure 1, each section of the line comprises shunt capacitance C and series inductance L both of which are simultaneously adjustable in the control line to vary the retardation time of the delay line. This circuit acts as a low pass filter. At frequencies well below its cut-ofif frequency, its performance is closely in accordance with the following equaphysical form of the where T=the delay per section in seconds, L=inductance per section in henries, C=capacitance per section in farads and, Z =characteristic impedance of the delay line in ohms. Thus, the cascaded sections form an unbalanced lumped parameter transmission line.

It is seen that if L and/ or C were increased or decreased in value, T would also increase or decrease, and further if L and C were increased or decreased together and kept in constant ratio, the characteristic impedance of the line would remain constant in spite of the change in delay.

The actual construction of the circuit of Figure 1, is shown in Figure 2. The arrangement comprises two series of fixed elements consisting of plates 2-2 etc., and fixed turns comprising sides 3, 4, 5, 6, 7, 8, 9, 10 and 11.

The turns 3, 4 and 5 are connected to the first plate 2, and the turns 6, 7 and 8 are connected to the second plate 2 forming two fixed sections of the stator. The rotatable section of the unit comprises plate 12-42. The first p ate 12 rotates between the plates 22 and the second plate 12 rotates between the second plate 2 and the next plate not shown in the figure.

The fixed elements comprising the strips 6, 7, 8, 9, and 11, form the inductance in the line, while the plates 22 form one of the capacitor plates of the line. There is one turn in each section of the line and when the elements 1212 are in a vertical position, both capacity and inductance are at a minimum.

In the position represented in Figure 2, the capacitance is approximately the maximum capacitance used in the delay line. The capacitance in the line itself would be largest when the plates 12-12 are substantially perpendicular to the plates 2-2. The elements 3, 4, 5, 6, 7, 8, 9 and 10 are elements of a repetitive structure which form the single turns for each inductance formed between the parallel plate elements. The elements 6 and 10 of each turn are the elements which carry under successive rotating plates to connect together the successive turns of inductance of the line.

The conductive rotor plates 12 are of course insulated from the other elements of the line. For this purpose the rotor plates 12 are mounted on a shaft 13 which is perpendicular to the axis of the plane of each plate. In this way the rotor plates 12 and the stator plates 2 function in a manner as a variable condenser. As the shaft 13 is rotated clockwise, the capacitance in the delay line as viewed in Figure 2 decreases in value and the values of series inductors drop as well, since the rotor plate becomes inserted between adjacent turns of each section. This is due to the fact that the insertion of the rotor plates between each inductor section decreases or partially eliminates the mutual inductance between the rotor turns.

In a preferred embodiment of the invention, the plates are given such a shape as to cause L and C to vary in equal proportions, thus keeping the characteristic impedance of the line constant.

In a typical structure, the delay can be made to vary over a ratio of 21 with a 45 rotation of the rotor shaft.

Figure 3 shows how the compensating elements are added to the basic delay line of Figure l in order to minimize phase distortion which would otherwise be present. As is characteristic of the helical wound delay line, such distortion comes from the mutual coupling between adjacent turns of the solenoid. At higher frequencies, the current in the coupling turns become increasingly out of phase, causing the effective inductance per turn to drop as the frequency is increased. This results in a drop in the delay at high frequencies. The'combination used as a preferable embodiment in the present invention consists of the addition of element L and C to each delay line section. Series L and C and shunt of L and C occur at a frequency considerably above the operating frequency range of the delay line. Accordingly the effect of compensation is to cause a rise of apparent inductance in the branch L and apparent capacitance in the branch C as the frequency increases. This results in an increase of delay with frequency which combines with the previously mentioned decrease to create nearly constant delay at all operating frequencies.

Figure 4 shows for a single section of the line, a physi cal realization of compensation described with reference to Figure 3. The inductance L is achieved by connecting each solenoid turn to its corresponding stator by the conductive element 14, which is a metallic strip extending from the stator plate 2 to the end of the side 5 of the inductor coil. This connection may be made long enough to have the required inductance for such compensation. The capacitance C is formed by making the stator plates considerably larger than they would otherwise need to be. The capacitance between adjacent stator plates (which shunt the solenoid turns) has an appreciable effect as a compensating means. This is shown in Figure 3 as C and in Figure 4, this capacitance comprises in part the strap 15 between the two sections 2 and 2 of the stator plate 2. The grounded rotor plate 16 is indicated in Figure 4 between the two sections of the stator plate 2 and 2 which in the vertical position, has a minimum amount of capacity between the stator plates and the rotor plate, the capacity comprising only the effect of the connecting strip 15 and the fringe effect of the grounded rotor plate and the stator plates.

It will be seen on examining Figure 4, that the variable part of the capacity C shown in Figure 3, comprises the capacity between the grounded rotor plate and the stator plate, and that the variable inductance comprises the mutual inductance between successive coil turns between which the grounded rotor plate passes to increase and decrease the mutual inductance. Besides this mutual inductance and the fixed inductance established by the turns 5 in itself, there is the inductance L as indicated in Figure 4 which comprises the fixed linkage element 14, shown in Figure 4. Each single turn of the inductance between successive rotor plates is connected to the preceeding and the following turns by a cross connection which extends diagonally at the bottom of the coil. This is shown by the element 17, Figure 4. By forming the stator and rotor plates as indicated in Figures 4 and 4a, which shows the rotor plate in the maximum and minimum delayed position respectively, a uniform ratio between L and C may be obtained. That is to say', as the products L and C increases, the ratio remains constant, so that the characteristic of the line will be constant even though the amount of delay is proportionately increased.

It will be seen from Figure 4a, that the minimum delay position, that is the position in which there is a minimum delay in the line, is when there is least capacity between the stator plate and the rotor plate and when also due to the interception of the stator plate between each successive turn of inductance, there is also a minimum inductance in each section of the delay line. In actual construction, delay lines have been used for compensation with 91 sections in a long line and 61 sections in a short line. This number of sections may be varied depending upon the application.

The characteristic impedance of the line may be approximately maintained constant as has been stated, and, values such as 70 ohms have been used for the impedance of the line. The inductance in the line shown in Figure 4, has been constructed in the form of a square helix of two inch cross sections which run nearly the length of the box in which the line is contained. At high frequencies, the neighboring turns carry currents whose phase differs somewhat from the phase in a given turn, thus decreasing the effective value of the mutual inductance. This effect might give rise to a phase distortion, but in the line shown in this application, this is corrected by the inductive link from the stator plate to the coil turn and by suitable position and design of the stator and ground rotor plate. As has been previously stated, the phase distortion is compensated for by two means indicated in Figure 3. The capacitance between adjacent stator plates C shunt each coil turn, causing effective inductance to rise with frequency. Further each stator plate is con ected to its coil turn by the inductive loop v circle was 117 feet in diameter.

L causing effective capacity to rise with frequency. Both effects tend to ofiset the drop in delay with increasing frequency that would otherwise be observed. These corrections may be made to maintain a constant impedance for each section of the line.

Figure 5 shows the array of antenna elements which are connected to a common transmitting or receiving center through the delay lines, the outer set of antenna elements being connected to the longer delay line, while the inner set of antenna elements are connected to the shorter delay line. The inner circle of antenna elements are indicated by the odd numerals 1a to 31a, and the outer set of antenna units are indicated by the even numerals, 2a to 32. It will be noted that in this antenna array, the inner units are staggered between the outer units. For instance, 1w lies directly angularly between- 3211 and 2a, and similarly, 31a lies directly between the unit 30 and 32a. In one array which has been set up, the inner circle was 77 feet in diameter and the other Where 32 units are used in the array system, each antenna will be spaced apart from the next angular spacing by 11%", and each antenna in each unit will be spaced apart from the next antenna in its unit by 22 /2 the system operates, let us assume a wave is approaching from the northerly direction, indicated by the arrow in the figure. In this case the wave will reach the unit 32a and 2a at the same time and then subsequent to that, it will reach the unit 30a and 4a, and then it will reach the unit 1a in the inner circle and subsequent to that the unit 310: and 3a, in the inner circle.

In order to make the received waves in phase for the various units, the units first receiving the wave will have to have a longer period of delay than the units last to receive the wave.

In the illustration in Figure 5, the compensation will be the time equivalent it would take the wave to advance on a linear front from one unit to the next unit. In the case of the array shown in Figure 5, all of the units will be compensated so that the delay in each unit is such as to make all the units in phase with the units last receiving the wave, namely the units 18a and 16a. This means that the maximum compensation necessary for the outer circles is equivalent to 117 feet in distance and that of the inner circle 77 feet in distance plus the difference in radii between the outer diameter and the inner diameter. The outer delay line will therefore have to accommodate a 117 foot delay and the inner delay 77, plus 20 feet delay, making 97 feet. This is readily accomplished in the arrangement of the present invention.

While the system in Figure 5 has been described as a receiving system, it is of course obvious that the same arrangement is used for transmitting system to transmit a beam in which all of the antenna units will be radiating in the same phase in the same direction so that the units form a plain wave front with a center of the circle of units as the center on which the diameter of radiation may be referred. As the delay line or the delay for each unit is varied progressively, the direction of radiation will be swung around a complete 360 so that the wave front of the radiated energy or the energy to be received will be normal to a given diameter of the circle. In the case of the received wave, the object is to adjust the delay in the lines so that the antennas will each be compensated for a particularly azimuth direction which will be the direction of the received wave. Where the direction of arrival of the received signal is known, the array will be compensated by setting control for the desired direction and where not known a maximum indicating design may be used.

Figure 6 shows a plurality of delay lines arranged to control simultaneously the amount of delay in each antenna element feed so that the radiating antennas may all have their phases such that their radiation in the desired directions are all in phase. The larger boxes 20 of Fig- In order to understand how ure 6, indicate the longer delay lines of the outer circle and the smaller boxes 21 indicate shorter delay lines of the inner circle. These are all mounted on a frame around a circumference in positions established by the positions in the array circles of the respective radiator. The frame is a fixed frame and may be constructed in any way to hold the units in a fixed and secure position. The control of each delay line is had through the end of the box by means of a drive and a linkage system operated from a central driving unit 23 by which all of the outer delay lines and inner delay lines are simultaneously controlled. The structure of the drive by means of which each delay line is controlled, is shown diagrammatically in Figure 7, and the actual connection and construction at the delay line itself is indicated more specifically in Figure 10, while the inside ring to which each linkage arm is attached is shown in Figure 8. The drive unit 23 at the center of the circle of delay lines has two two drive rings, an upper drive ring and a lower drive ring. The lower drive ring operates the larger delay lines and the upper drive ring operates the shorter delay line. This arrangement is indicated in Figure 12, where the drive units and the connecting linkage element are indicated. The simple three bar linkage system used in this arrangement is indicated diagrammatically in Figure 7. It is seen that when R the linkage bar at the outer end of the linkage L is somewhat larger than R the bar connected to the driving ring, that a continuous rotation of R, from the point A about the center C would cause the point B to move back and forth in a short are centered at C which is a pivot on the outer end of the system. The linkage indicated in Figure 7, is such that the point C and C are the centers of rotation of the inner drive and the rotation of the delay line rotors respectively. There are three independent adjustments which can be made on the linkage drive of each delay line. First the adjustment of the angle 0 between the arms R and R secondly the adjustment of the radius R by turning the radius screw shown in Figure 10, as numeral 26, and by the adjustment of the long link L by turning the link arm itself on the screw 27, Figure 10, after loosening the link arm lock 28, which clamps the link arm to the screw 27. The adjustment of the angle 0 is made by turning the angle screw which is shown at 29, Figure 10. When these controls in Figure 10 are properly adjusted, the delay of the corresponding line will vary in the correct manner as the drive units operate. There are positions for locking these controls and they are normally left locked.

In Figure 10, the point of pivot C of delay line may be seen at the right end of the crank R and the fixed crank is indicated at R beneath the crank R just showing out to the left side. An angle lock nut 30 is used to lock together the two arms R and R so that these may be fixed together at the desired angle 0 for proper adjustment. As the crank R is rotated by means of oscillatory motion 'of the drive ring 25, Figure 8, the crank R will turn through a whole 360 whereas the other end of the link, the arms R and R will turn through a desired angle which will encompass a maximum delay in the delay line. As has been previously mentioned, there are two drive rings, the upper drive ring 25 and the lower drive ring 24. Only one drive ring is shown in Figure 8, and this as diagrammatically illustrated, may correspond to each of the drive rings shown in Figure 6.

As will be seen in Figure 8, each drive ring rests on; three small cranks or swivels, 31, 32, and 33, the mechanics of the arrangement being such that the swivels must rotate in unison and the ring is constrained to move in a circular translational manner; that is, every point on the ring moves parallel to every other point, each point describing the proper circle which in one of the constructions employed by the applicant was .750" radius. The 16 points of linkage attachment are equally spaced around the drive ring 24 (25), and the delay lines driven therefrom are correspondingly located on the mounting frame.

In this way the phase at which each delay line shaft is rocked back and forth bears the correct relationship to the placement of the respective radiators in the array circle. The dashed ring 34 shown in Figure 8 indicates the rotational movement of the point A about a center for each of the delay lines. Since there are 16 delay lines attached to each of the rings, these will be spaced equally apart on the ring. One of the three drive swivels, 31, 32, 33 is directly driven, the other two serve as slave swivels, and serve only to guide the ring motion properly. The two drive rings 24 and 25 have a common driven swivel, so that all 32 links may be driven from the same driving motor. While one drive ring could be used for all 32 linkages, it is preferable to use two drive links to minimize vibration. The two rings rotate in the same direction but are always on opposite sides of their circular travel. The inertial reaction transmitted to the ring by the link arms is consequently opposite in direction for the two rings and net force acting on the drive unit, as a whole, is thereby reduced to a very small value. The drive rings are covered by a spun aluminum dust cover as indicated in Figure 6.

When the system is used for transmission, the antenna elements are connected to the delay line and then to an RF transformer, Figure 13, which is positioned in the antenna house at the center of the circular array. The unit comprises an RF transformer with a 50 ohm input and a 2.2 ohm output. This RF transformer has an RF transformer circuit as indicated in Figure 9, where the input comprises a group of turns 40 with the secondary comprising the turn 4-1 connected as an auto transformer in the primary circuit. The ratio of transformation in the transformer is to l. The input connection is made by feeding all turns of the primary which in the specific arrangement used consisted of five turns, with the 2.2 ohm output taken across only one turn. The transformer input circuit consists of a series inductance L a capacitance C; and a shunt capacitance C connected across the transformer proper. In the output circuit a capacitance C is connected across the output terminal. As indicated in Figure 13, the transformer proper is an axially wound strip of copper. The resultant cylindrical shape unit 50 is about long and 6 in diameter for the arrangement described in the present invention providing the necessary close coupling for a broad band transformer. The outer turn 51 comprises the secondary turn of the transformer, while the inner turns 52 are the primary turns of the transformer. The terms primary and secondary presuppose use of the system for transmission. The use of a wide copper strip as a secondary turn is required tomaintain a reasonable current density at a low impedance value such as 2.2 ohms. Parallel circuits from the output of the transformer go into each delay line and through the delay line to the antenna unit. As indicated in Figure 13, the outer turn 51 of the transformer terminates in a broad strip 51 to which each of the thirty-two lines described in Figure 6, is connected. The circuit of Figure 13, corresponds to that of Figure 9, the inductance L, of Figure 9, corresponding to the same lettered parts and 41 and 40 corresponding to 51 and 52 of Figure 13.

Having now described my invention, I claim:

1. An electromagnetic radiating system for operation over a relatively wide frequency range centered about a mean frequency comprising, a plurality of high frequency antenna elements, a like plurality of lumped parameter delay lines each forming an unbalanced transmission line for coupling a respective one of said antenna elements to common means exchanging high frequency energy with said elements, each of said delay lines comprising a plurality of cascaded sections with each section including a main inductance in series with the main inductance of adjacent sections, a main capacitance formed by parallel rotor and stator plates with one of said plates coupled to the junction between series main inductances of adjacent sections by an auxiliary inductance section and the other connected to a common line of said delay line, auxiliary capacitance elements connected between said one plate of adjacent sections, and means for displacing said rotor with respect to said stator to alter said main inductance and main capacitance while maintaining the ratio therebetween unchanged to vary the delay furnished by said delay line without altering its characteristic impedance.

2. An electromagnetic radiating system for operation over a relatively wide frequency range centered about a mean frequency comprising, a plurality of high frequency antenna elements, a like plurality of lumped parameter delay lines, each forming an unbalanced transmission line for coupling a respective one of said antenna elements to common means exchanging high frequency energy with said elements, each of said delay lines comprising a plurality of cascaded sections with each section including an inductance in series with the inductance of adjacent sections, a capacitance formed by parallel rotor and stator plates coupling the junction between adjacent sections to a common line of said delay line, said rotor plates being movable in planes between adjacent inductance sections, and means for displacing said rotors with respect to said stators to alter said inductance and capacitance of each section while maintaining the ratio therebetween unchanged to vary the delay furnished by said delay line without altering its characteristic impedance over said frequency range.

3. In a high frequency radiating system, a plurality of delay lines for exchanging energy with a like plurality of radiating elements, each delay line comprising a lumped parameter transmission line including a plurality of cascaded sections each having parallel rotor and stator plates forming a shunt capacitance and a conducting loop substantially coplanar with and connected to a stator plate and connected in series with the conducting loop of an adjacent section to form a series inductance, and a common rotatable shaft supporting each rotor plate with respect to adjacent stator plates and conducting loops so that the rotation of said shaft varies the inductive coupling between each rotor plate and adjacent stators and the electrostatic coupling between rotor and stator to vary said series inductance and said shunt capacity while maintaining the ratio therebetween substantially constant, said delay lines being arranged on the circumference of a circle with said common rotatable shafts being perpendicular to the plane of said circle, a drive ring rotatably supported in the center of said circle and in the plane thereof, and means for coupling the angular motion of said ring to angular motion of each of said rotatable shafts.

4. Apparatus in accordance with claim 3 and further comprising a second plurality of said delay lines arranged upon the circumference of a second circle concentric with said first circle with said common rotatable shafts of said second plurality of delay lines perpendicular to the planes of said circles, a second rotatably supported drive ring inside of said circles and concentric about their common axis in a plane parallel to but spaced from said first drive ring, and means for coupling angular motion of said second drive ring to angular motion of said second plurality of common rotatable shafts.

5. Apparatus in accordance with claim 4 and further comprising means for moving said first and second drive rings in synchronism.

6. Apparatus in accordance with claim 3 and further comprising, a circular array of radiating elements disposed upon 'a circle surrounding said plurality of delay lines, means for coupling the output of each delay line to a respective radiating element, and means for coupling the inputs of all said lines in parallel.

7. Apparatus in accordance with claim 4 and further comprising a circular array of radiating elements disposed upon first and second circles one of which is inside the other, both being outside said first and second plurality of delay lines, means for coupling the output of each of said first plurality of delay lines to a respective radiating element on said first circle, means for coupling the output of each of said second plurality of delay lines to a respective radiating element on said second circle, and means for coupling the inputs of all said delay lines in parallel.

8. Apparatus in accordance with claim 3 wherein said means for coupling angular motion comprises, a three bar linkage associated with each delay line, said linkage comprising, a first bar extending radially outward from said rotatable common shaft, a second rotatably mounted bar, means responsive to angular motion of said driving ring for imparting rotary motion to said second bar, a third bar pivotally connected at each end to the ends of said first and second arms, the length of said first arm being greater than that of said second arm and much less than the length of said third arm.

9. Apparatus in accordance with claim 8 and further comprising, means for adjusting the angular orientation of each first arm with respect to the associated common rotatable shaft, means for adjusting the length of each 10 first arm, and means for adjusting the length of each third arm.

10. Apparatus in accordance with claim 7, and further comprising a source of high frequency energy, and a step down transformer for coupling said source to said parallel connected delay line inputs.

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